U.S. Pat. No. 3,966,410 basically describes an analytical separation method known as solid phase extraction.
In the solid phase extraction system, one provides an elongated generally cylindrical tube having an inert chemically non-reactive packing material therein, which has a large surface-to-volume ratio, and a small lower outlet in the form of a nozzle. A liquid sample is introduced into the tube through an upper input end. When the sample reaches the packing material, it spreads out as a thin liquid film with a large surface of the packing material and forms thereon in a stationary state. The primary function of the packing material is to provide a large surface area over which the sample spreads out as thin liquid film, without reacting wiht the packing material. The entire sample remains on the packing material as a thin liquid film, so long as no other matter is introduced into the tube. To extract a compound or a family of compounds from the thinly spread out sample, a solvent system is introduced into the tube. The term "solvent system" refers to a solution of a solvent or solvents which is immiscible in the original sample liquid and one or more reagents selected on the basis of the desired extract. This solvent system, commonly referred to as an "eluate", is utilized to elute (liberate) out the compounds of interest from the sample.
As the eluate passes through the packing material, it comes in contact with the thinly spread out sample over the very large surface area and intimate contact between the sample and the solvent system is achieved. The reagents are chosen to cause the partition coefficients of only the desired compounds to be extracted to change in favor of the eluate. Thus as the eluate comes in contact with the spread out sample, the desired compounds become dissolved in the eluate and exit therewith as the desired extract. This extract is known as the "eluent". The last of the compounds in the sample remain dissolved in the sample liquid and remain in a stationary state on the packing material. After extracting on extract, a subsequent extract consisting of a different family of compounds may be extracted from the remaining sample by passing a second eluate through the tube. The second eluate causes the partitions of coefficients of the compounds to be included in the second extract and second eluent. This process may continue with different eluates until the subject of the investigation is isolated in the packing, and may further be utilized to detemine from the various eluents what has been extracted.
The packing materials may include cellulose fibrous materials, such as absorbent cotton, filter paper, and multi-layered pressed alpha-cellulose sheets, glass fiber materials such as glass wool or pyrex wool and silica sand or gel in a powdered or fine granular form. The packing material used should be of high purity so as not to contaminate the sample and posses a high degree of absorbency in order to absorb and thereby retain the sample therein in a stationary state. Moreover, it should have a high ratio of surface area to volume so as to enable the sample to spread out as a thin film over the packing material surface and enable the eluate to pass therethrough.
The eluates are chosen to change the partition coefficient of the compounds to be extracted from the sample in favor of the eluate. Therefore, the desired compounds or the sample separates from the sample and become dissolved in the eluate. The packing material is practically saturated by the sample and therefore none of the eluate is retained by the packing material, and also therefore, the eluent flows through the packing material and exits the tube through the lower nozzle end.
In this solid phase extraction process, the pH of the sample may be adjusted to change the partition coefficient in a direction to enable the compound(s) to be extracted to become soluble in the eluate.
The solid phase extraction technique may investigate liquid samples such as blood, urine, bile, stomach content, homogenized tissue, liquid or liquified foods and beverages, waste water or water for human and animal consumption, and industrial preparations, only by way of example. The chemical compounds contained in such liquid samples may be drugs and drug metabolites, hormones, lipids, metabolites, normal or abnormal body metabolites, hormones, lipids, vitamins, poisons and contaminants useful and deleterious. The elutes may be any materials which may be utilized to remove compounds from the samples in an attempt to isolate a substance under investigation.
The solid phase extraction process is considered to be an improvement or an alternative to what is known as the "liquid-to-liquid" extraction process, which is also described in U.S. Pat. No. 3,966,410.
Besides the solid phase extraction process described above, there are a number of processes that separate compounds one from another. These processes range from simple filtration methods to complex extraction procedures. In even the relatively simple filtration methods where components are separated on the basis of size, such processes can become complex within a large variety of filter media available. Extraction procedures range from simple methods where a compound is washed with only one solvent or elute, up to complex methods with a variety of solvents or elutes in a sequential manner. In general, all these methods involve the use of some type of container to which the sample under study is added. The container will have some method of holding or supporting the samples, where it is subjected to the flow of additional eluates, and a means as necessary to collect the eluent. The solvents or eluates are normally liquid and require some method of forcing them through the sample and the filtering means. While gravity is always available, this is a slow process and usually either pressure or vacuum is required to overcome the frictional flow losses in the support medium. These separation techniques might further be subdivided into different categories. One category is where different samples or eluates are passed through a reusable substrate to obtain separation. An example of this is high performance liquid chromatography (HPLC) where the passage of the sample through the column achieves the desired separation. One eluate after another is passed through the same column. To hasten this type of process, it is only necessary to provide a means of injecting the various eluates into the column.
All of these known processes of separation are primarily quite time consuming, and are primarily manually accomplished.
Accordingly, the present invention is directed to separation techniques in which a virgin substrate is used for each separation, for example, where a filtration process requires a new filter medium between each sample. However, the silica gel or sand may be cleaned and reused. Due to the number of steps that may be required, these processes are usually manually performed. The means of adding solvents or catching eluates may be mechanized, but the changing of substrates between samples is usually done manually. While the method of processing an individual sample may be mechanized, the changeover from one sample to another requires manual interface.
Accordingly, the present invention provides a new and improved process and apparatus for speeding the processes for separating compounds for the analytical study of samples.